1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor that prevents the generation of a reddish effect in an output image.
2. Discussion of the Related Art
Image sensors are semiconductor devices for converting an optical image to an electrical signal and include CMOS image sensors having a number of metal-oxide-semiconductor (MOS) transistors, corresponding to the number of pixels, integrated on a single chip with peripheral circuitry. The circuitry sequentially outputs the electrical signals of the MOS transistors. CMOS image sensors use complimentary MOS technology to decrease feature size, power consumption, and fabrication costs and are applicable to such products as digital cameras, cellular telephones, personal digital assistants, notebook computers, bar-code readers, and toys. A CMOS image sensor largely includes a signal processing chip which includes an array of photodiodes provided with an amplifier, an analog-to-digital converter, an internal voltage regulator, a timing generator, and digital logic circuitry.
To enhance the photosensitivity of a CMOS image sensor, its fill factor may be improved; that is, the photodiode area is increased with respect to the area of the device itself. Increase of the fill factor is limited, however, by the presence of the associated logic and signal processing circuitry of each photodiode. Enhanced photosensitivity may also be achieved by focusing incident light, which is otherwise deflected, onto, for example, a microlens provided to each photodiode. The microlens should concentrate the incident light onto the photodiode and away from the adjacent areas where there is no photodiode surface.
An image sensor used for, as an example, a mobile phone, is included in an optical module, which is compactly fabricated according to a viewing angle (AOV) of typically about 55° to 65°, such that the light from a light source or object is obliquely incident to the edges of an image area, typically at an angle of 30° or less, as shown in FIG. 1. The angle of view determines the size of each color filter of a color filter array as well as their corresponding microlenses, all of which are smaller toward the center of the image area of the array. That is, direct light that is incident at the center of the image area may concentrate a visible light ray onto the corresponding photodiode, but the same light source may be obliquely incident at the edges of the image area, such that concentration differs according to color pixel due to a difference in refractivity per wavelength. That is, the refractivity of a ray of red light is smaller than the refractivity of a ray of green or blue light. This refractivity differential results in an output differential between an image area center and its edges, thereby imparting the generated image with an unwanted reddish hue.